Tuning the Metal-Free Room Temperature Phosphorescence of Fluorene-Based Chromophores through Side-Group Molecular Engineering

A series of bromofluorene-based metal-free dyes (BrFX) exhibiting room-temperature phosphorescence (RTP) were synthesized via a scalable two-step process in high yields from the linkage of 2-bromofluorene (BrF) through a ketone with different side group (X). Structural analysis of BrFX revealed that the ketone group was well conjugated with the fluorene group but less conjugated with the X side group. The nature of the X side group played a crucial role in fine-tuning emission maxima of BrFX. Moreover, incorporating aromatic side groups having low triplet energy effectively red-shifted RTP and increased its lifetime. Theoretical calculations using density functional theory revealed that the highest occupied molecular orbital was localized on the fluorene core, supporting these experimental observations. BrFX showed strong phosphorescence in diverse amorphous semiconducting hosts with quantum yields up to 65%, enabling their application in light-emitting electrochemical cells. These findings underscore the success of converting BrFH into its ketone analogs, BrFX, as an effective strategy to enhance both phosphorescence and electroluminescence, representing a notable advancement in RTP molecule design.